The present invention generally relates to fabrication of semiconductor devices, and more particularly to a method for exposing a semiconductor pattern on an object such as a semiconductor substrate by a charged particle beam.
In the fabrication of a submicron semiconductor device that has a pattern width typically in the order of 0.1 .mu.m, the charged beam exposure system that uses a high energy charged particle beam such as an electron beam is essential. In the electron beam exposure system, the device pattern is exposed on the surface of a semiconductor wafer by a finely focused electron beam and one can easily achieve a pattern size of less than 0.05 .mu.m with an alignment error of 0.02 .mu.m or less. On the other hand, a drawback of such a charged particle beam exposure system is in that the throughput of exposure is limited because of the fact that the device pattern is written by a single stroke of the focused charged particle beam, in contrast to the conventional optical exposure system that exposes a large area of the substrate by a single shot of the optical radiation.
In order to overcome the foregoing problem, the inventor of the present invention has previously disclosed in U.S. Pat. No. 5,051,556, an electron beam exposure system that patterns the shape of the focused electron beam according to the elemental device patterns that form a part of the large and complex pattern of the semiconductor device. There, a mask called a block mask is provided in the electron beam exposure system in correspondence to optical path of the electron beam for shaping the electron beam upon passage therethrough, and the mask is provided with a plurality of apertures shaped according to the elemental device patterns. By deflecting the electron beam to hit one of these apertures, one can shape the electron beam according to the shape of the aperture and pattern having the shape of the selected aperture is on the surface of the wafer or substrate. By repeating the exposure of the elemental device patterns consecutively, one can obtain the desired large and complex pattern of the semiconductor device. Such a so-called block exposure system is particularly useful in the exposure process of device patterns that include a repetition of predetermined elemental device patterns, as in the case of semiconductor memories.
FIG. 1 shows the construction of the electron beam exposure system proposed previously by the inventors of the present invention.
Referring to FIG. 1, the electron beam exposure system includes an optical system 10 for carrying out the exposure by an electron beam and a control system 50 for controlling the exposure operation.
The optical system 10 includes an electron gun 14 that has a cathode electrode 11 for emitting thermal electrons, an anode electrode 13 for accelerating the electrons to form an electron beam and a grid electrode 12 for controlling the flow of the electrons from the cathode electrode 11 to the anode electrode 13. The electron beam produced by the electron gun propagates as a divergent beam generally in the direction of a target or object W, held on a movable stage 35.
The divergent electron beam thus produced is then passed through a shaping aperture formed in a plate 15 to have a desired cross section and directed to travel in coincidence with a predetermined optical axis O that extends from the cathode electrode 11 to the object W on which a pattern is to be written. Typically, the object W is a semiconductor substrate. In order to achieve the desired alignment of the electron beam with respect to the optical axis O, adjustment coils 36, 37, 38 and 39 are provided along the optical axis O.
The electron beam thus shaped by the aperture plate 15 is then focused on a point P1 located on the optical axis O by a first electron lens 16. A deflector 17, corresponding to the point P1, that deflects the electron beam in response to a control signal HS1 supplied, thereto that the electron beam hits a block mask to be such described later.
The electron beam is then received by a second electron lens 18 that has a focal point coincident with the point P1 and is converted to a parallel electron beam. This parallel electron beam is then passed through a block mask 20 on which a plurality of apertures for shaping the electron beam are formed. Upon passage through a selected aperture, it is possible to shape the electron beam such that the electron beam has a desired cross section.
In order to address the apertures on the block mask 20, a pair of deflectors 21 and 22 are provided above the mask 20 such that the parallel electron beam is initially deflected away from the optical axis O and then deflected back to travel in the direction parallel to the optical axis O. Further, in order to return the electron beam in coincidence with the optical axis O, another pair of deflectors 23 and 24 are provided below the mask 20. It should be noted that the deflectors 21-24 cause the foregoing deflection and hence the addressing of the selected aperture in response to deflection control signals PS1-PS4 that are supplied thereto.
The parallel electron beam thus returned to the optical axis O is then passed through a third electron lens 19 and focused at a point P2 located on the optical axis O. The electron beam is then passed through a demagnification optical system including a fourth electron lens 26 and another electron lens 29, and focused on the object wafer W by an objective lens 32. Thereby, the image of the selected aperture on the mask 20 is projected on the upper surface of the wafer W.
In the objective lens 32, various coils and deflectors such as a dynamic focusing coil 30 and a dynamic stig coil 31 are provided for focusing correction and astigmatic correction. Further, deflectors 33 and 34 deflect the focused electron beam on the object wafer W such that the electron beam moves over the object wafer W, wherein the deflector 33 causes movement of the beam spot within a maximum area of about 2 mm.times.2 mm, while the deflector 34 is used to deflect the electron beam within an area of about 100 .mu.m.times.100 .mu.m. Furthermore, a refocusing coil 28 for additional focusing control is provided above the electron lens 29.
In order to control the turning on and turning off of the electron beam on the object wafer W, a blanking aperture plate 27, provided between the fourth electron lens 26 and the fifth electron lens 29, which passes the electron beam through a blanking aperture 27a therein in coincidence with the optical axis O. The blanking aperture 27a has a reduced diameter and the blanking aperture plate 27 interrupts the electron beam when the electron beam is deflected away from the optical axis O. Thereby, the electron beam disappears from the surface of the object wafer W.
In order to effect such a turning-on and turning-off of the electron beam, a blanking deflector 25, provided between the lens 19 and the blanking aperture plate 27, deflects the electron beam away from the optical axis O in response to a blanking control signal SB supplied thereto.
Next, the control system 50 for controlling the optical system 10 will be described.
Referring to FIG. 1 again, the control system 50 includes a magnetic storage device 51 that stores various design data of the semiconductor device or integrated circuit to be formed on the object wafer W and a CPU 52 that controls the optical system 10. The CPU reads the pattern information of the semiconductor device such as the pattern data, the positional data indicating the location on the wafer W on which the pattern data is to be written, the mask information indicating the array of the apertures on the block mask 20, etc. The pattern information and the mask information thus read out from the storage device 51 are transferred on the one hand to a data memory 54 and on the other hand to a sequence controller 62 via an interface circuit 53.
The data memory 54 stores the pattern information and the mask information and transfers the same to a pattern generator 55 that generates deflection control data PD1-PD4 in response to the pattern information and the mask information supplied thereto. The deflection control data PD1-PD4 are sent to a D/A converter 57 where they are converted to the analog deflection control signals PS1-PS4. Thereby, the selection of the apertures on the mask 20 is achieved as already described. The pattern generator 55 further produces positional data SD3 indicative of the location of the object wafer W on which the exposure is to be made. The data SD3 is sent to a D/A converter 65 where it is converted to an analog signal S3 that drives the sub-deflector 34 in the objective lens 32.
The pattern generator 55 further produces correction data HD indicative of the difference in the desired pattern and the selected pattern on the mask 20 and supplies the same to a D/A converter 56 where the data HD is converted to a control signal HS1 that drives the deflector 17. In response to the control signal HS1, the electron beam is moved over the block mask 20 and additional shaping is achieved by offsetting the electron beam slightly from the selected aperture. For example, the variable shaping of the electron beam is achieved by the deflector 17. Further, the deflector 17 achieves the deflection of the electron beam at high speed and in a limited area typically having a size of 500 .mu.m.times.500 .mu.m. The deflectors 21-24 are used, on the other hand, for the deflection of the electron beam for a relatively large area typically the size of about 5 mm.times.5 mm, though with reduced speed. Typically, the deflector 17 is constructed with an electrostatic deflector while the deflectors 21-24 are constructed with electromagnetic deflectors.
Additionally, the pattern generator 55 produces control data MKD for moving the block mask 20 and supplies the same to a mask drive mechanism 58. The mask drive mechanism 58 moves the mask 20 in response to the control data MKD in a plane substantially perpendicular to the optical axis O. Thereby, the entirety of the apertures on the mask 20 can be addressed by the electron beam by moving the mask 20 such that the specified aperture moves into the area where the addressing can be achieved by the deflection of the electron beam. Further, the pattern generator 55 produces a control signal for driving the refocusing coil 28 and supplies the same to the refocusing coil 28 via a D/A converter 70. Thereby, the proper focusing on the surface of the object wafer W is maintained even when the electron beam is deflected by the deflectors 33 and 34.
The pattern generator 55 produces a timing signal T for indicating execution of the exposure or waiting for the exposure. The timing signal T is supplied to a clock control circuit, or generator (CLOCK GEN) 59 that in turn produces blanking control data BC for indicating the interruption of the exposure. The data BC is then supplied to a blanking control circuit 60, and the blanking control circuit 60 drives the deflector 25 via a D/A converter 61 that produces the blanking control signal SB described previously. The clock generator 59 further produces a system clock, running at a predetermined rate, that determines the throughput of exposure as will be described in detail later.
The sequence controller 62 detects the timing information transferred thereto from the interface circuit 53 and indicating the commencement of the exposure process, and controls the data memory 54 via the pattern generator 55 to output main deflection data MD that is supplied to a deflection control circuit 63. The deflection control circuit 63 produces main deflection control data SD2 in response to the data MD supplied thereto and supplies the data SD2 to a D/A converter 64 where the data SD2 is converted to a deflection control signal S2. This deflection control signal S2 drives the main deflector 33 in the objective lens 32. Further, the deflection control circuit 63 controls a stage position correction circuit 68 in response to the activation thereof by the sequence controller 62, and the stage position correction circuit 68 drives the deflector 34 via the D/A converter 56 that produces the drive signal S3 as described previously.
In cooperation with the deflection control circuit 63 and the stage position correction circuit 68, the sequence controller 62 activates a stage moving mechanism 66 for moving the stage 35 while monitoring the position of the stage 35 by a laser interferometer 67. Thereby, the exposure of a selected pattern on the mask 20 is made at any desired location on the object wafer W.
FIG. 2 shows the construction of the mask 20.
Referring to FIG. 2, it will be noted that the mask 20 is provided with a number of pattern areas E.sub.1 -E.sub.9 arranged in rows and columns and separated from each other by a pitch EL, wherein each pattern area may have a size of typically 5 mm.times.5 mm. Each area in turn includes a number of block areas B.sub.1 -B.sub.36, arranged in rows and columns as shown in FIG. 3, wherein each block area is separated from each other by a pitch BL and includes therein an aperture for shaping the electron beam. Typically, each block area has a size of 500 .mu.m.times.500 .mu.m.
FIG. 4 shows an example of the shaping apertures formed in the block areas B.sub.a -B.sub.d that are included in the foregoing block areas B.sub.1 -B.sub.36, wherein it will be noted that the block areas B.sub.a -B.sub.d contain apertures 20a-20g, wherein the pattern represented by these apertures require a plurality of shots when exposed by the rectangular beam. By deflecting the electron beam selectively by the deflectors 21-24, one can direct the electron beam to hit one of these block areas as indicated in FIG. 5, wherein the hatched region 71 represents the region illuminated by the deflected electron beam. In the illustrated example, the electron beam is shaped in accordance with the cross-shaped pattern corresponding to an aperture 72a. It will also be noted that FIG. 5 shows other block apertures such as apertures 72b-72d.
On the other hand, when it is necessary to write a pattern that is not included in the apertures on the mask 20, the electron beam 71 is directed to a large, generally square aperture 73a as indicated also in FIG. 5. There, by adjusting the overlap of the electron beam 71 with the aperture 73a, one can produce a shaped electron beam having a desired cross section. This process is known as the variable beam shaping. Such a variable beam shaping is used for exposing those patterns of which frequency of exposure is low and hence not provided on the mask 20 in the form of the shaping aperture.
When the variable beam shaping process is applied to the electron beam to produce a very fine, elongated beam shape with a pattern width of less than 0.1 .mu.m in the electron beam exposure process of FIG. 2, there arises a problem of decrease in the throughput of exposure. It should be noted a very high precision for the deflection of the electron beam is required to produce such a very fine beam shape, while such a high precision deflection requires a high precision D/A converter 57 for activating the deflectors 21-24. Associated with the use of the high precision D/A converter 57, the time needed for converting the deflection data PD1-PD4 to the corresponding analog signals PS1-PS4 increases substantially.
Further, it should be noted that such a variable beam shaping process, as applied to produce such a very fine elongated electron beam shape, causes a problem in that, while one of the elongated or principal edges of the beam thus obtained is shaped at the aperture 73a of FIG. 5, the other elongated beam edge is shaped by the shaping aperture provided in the aperture plate 15 of FIG. 1. Thereby, the focusing state of the electron beam may be different between the first principal edge and the second principal edge when the focusing action of the electron lens deviates from an ideal state. Further, because of the fact that the mask 20 interrupts most of the electron beam at the variable shaping aperture 73a, there is a tendency for a carbon deposit to accumulate on the upper major surface of the mask 20 during the operation of the system. As such a deposit of carbon accumulates asymmetrically on the one side of the aperture 73a that is illuminated by the electron beam, such a deposition modifies the distribution of the electric field in the vicinity of the variable shaping aperture and causes an unwanted distortion of the shaped electron beam. It should be noted that such a minute distortion of the beam shape can become detrimental to the device to be fabricated, particularly when the pattern to be exposed corresponds to a critical area of the device such as a gate region of a high speed MOS transistor or HEMT.